1. Field of the Invention
The invention relates generally to aerogel composite materials and methods of making them. In particular, the invention relates to an aerogel structure having an electrically connected network of ruthenium dioxide deposited throughout the structure and to methods of making the composite.
2. Background of the Related Art
Ruthenium dioxide (RuO2), one of the platinum group metal oxides, is an important industrial material due to its metallic electrical conductivity (RuO2 single crystal conductivity approaches 105 S/cm at 25xc2x0 C.) along with its excellent chemical and thermal stability and diffusion barrier properties. These characteristics have led to the use of ruthenium dioxide in electrodes for catalysis, electrolysis, photovoltaic devices, capacitors, thick and thin film resistors, etc.
Many techniques based on chemical vapor deposition (CVD) have been developed for depositing dense RuO2 films on flat substrates, including: sputtering or evaporating ruthenium metal in the presence of oxygen; plasma decomposition of Ru-bearing gases by glow discharge; thermal or photolytic decomposition of one of several organometallic precursors. Deposition by reacting oxygen with evaporated metal vapor can be activated by applying a dc current or r.f. radiation, as described in U.S. Pat. No. 5,055,319 to Bunshah et al. In Yuan et al. xe2x80x9cLow-Temperature Chemical Vapor Deposition of Ruthenium Dioxide from Ruthenium Tetroxide: A Simple Approach to High-Purity RuO2 Filmsxe2x80x9d Chem. Mater. 5 (1993) pp 908-910, incorporated herein by reference, the deposition of RuO4, which spontaneously reduces to RuO2, by CVD is described. The precursor was either RuO4 in a solution of water, pentane or carbon tetrachloride or pure RuO4 solid. Using this approach, RuO2 films 1-micron thick with resistivities of about 10xe2x88x922 ohm-cm were prepared.
For many RuO2 applications such as catalytic and sensing applications, it is desirable that the RuO2 material have the highest possible surface area in order to maximize the number of reaction sites. Conventionally, porous RuO2 electrodes are prepared by dip-coating a substrate in RuCl3 solution and heating in air to decompose the salt to RuO2. A technique for increasing the porosity of RuO2 by doping the ruthenium chloride solution with lanthanum chloride and, after firing, removing the lanthanum oxide by dissolving in sulfuric acid is described in Takasu et al., J. Alloys Comp. 261 (1997) p. 172, incorporated herein by reference. The RuO2 is stable and is five times xe2x80x9crougherxe2x80x9d than the sample prepared without La doping. These materials have good electrical conductivity, but the surface area is still fairly low.
Aerogels are a class of materials typified by extremely high surface area (up to 1000 m2/g) and porosity (up to greater than 99%). These properties are generally achieved by extracting the solvent from the pores of a wet porous gel under supercritical conditions, thereby avoiding shrinkage caused by capillary forces that develop during ambient drying. Although a wide range of aerogel compositions are possible, silica is the most widely studied. When formed by catalyzed hydration and polycondensation of a metal alkoxide solution, followed by exchange of pore-filling solvent with, and then removal of, supercritical carbon dioxide, silica forms a relatively robust monolith with extremely low electrical and thermal conductivity.
Efforts have been made previously to develop techniques to deposit Ru oxide on porous substrates. U.S. Pat. No. 4,298,439 to Gafney, incorporated herein by reference, claims a process for adsorbing RuCl3 in aqueous solution in/on a porous glass and then oxidizing in air at 120xc2x0 C. for one week to obtain the oxide. There is no indication whether this process resulted in a conductive film. Miller et al, J. Electrochem Soc. 144 (1997) L309, incorporated herein by reference, discloses a method of depositing Ru oxide by heating a volatile organometallic Ru compound in the presence of carbon aerogel in a sealed reactor. Decomposing the deposited organometallic by heating in flowing argon resulted in 2-nm Ru particles dispersed throughout the aerogel pores. The Ru/carbon aerogel composite had significantly higher specific capacitance than the untreated aerogel, but the Ru phase did not form its own electrically conductive network.
It is an object of the present invention to provide an electrically conducting structure having a high surface area.
It is a further object of the present invention to provide a method of forming an electrically connected deposit of RuO2 throughout an aerogel.
It is a further object of the present invention to provide a method of forming an electrically connected deposit of RuO2, wherein the method does not require high temperatures.
These and other objects are achieved by an electrically conducting composite made by a method comprising the steps of providing an aerogel structure, exposing the aerogel structure to a mixture of RuO4 and a nonpolar solvent in an inert atmosphere, wherein the mixture is held initially at a first temperature that is below the ambient temperature and below the temperature at which RuO4 decomposes into RuO2 in the nonpolar solvent and in the presence of the aerogel, and allowing the mixture to warm to a second temperature that is above the temperature at which RuO4 decomposes to RuO2 in the nonpolar solvent and in the presence of the aerogel, wherein the rate of warming is controlled so that as the mixture warms and the RuO4 begins to decompose into RuO2, the newly formed RuO2 is deposited throughout the aerogel structure as an electrically connected conductive deposit.
The aerogel structure of the present invention can be any conventionally known aerogel material. Preferably, the aerogel structure is made of a nonconducting material, such as silica. Typically, the aerogel structure is a silica aerogel prepared by acid- or base-catalyzed hydration and condensation of a metal alkoxide, tetramethoxysilane (TMOS), followed by washing to replace the pore liquid with acetone and then drying under supercritical CO2. The resulting monolithic aerogel consists of microporous (less than 2 nm pores) clusters that are about 10 nm in diameter, connected in a three-dimensional mesoporous (2-50 nm pores) network. The as-dried material has a surface area of about 800 m2/g. In order to strengthen the aerogel to allow refilling of the pores by a pentane solution, the aerogel is partially densified by sintering, typically at 900xc2x0 C. After sintering, the mocropores are gone, and the partially densified aerogel is about 80% porous with a surface area of about 400-500 m2/g. Collapsing the micropores within the silica domains provides a material that still has an ultra-high surface area, but does not have an extensive microporous area that would trap and isolate a deposited material.
To create an electrically connected deposit of RuO2 throughout the aerogel, the aerogel is exposed to a mixture of RuO4 and a nonpolar solvent. A nonpolar solvent such as pentane is preferred over an aqueous or nonpolar solvent because it has a lower surface tension, which minimizes capillary forces during re-wetting and re-drying of the aerogel at subcritical conditions. The mixture is initially kept at a temperature below the ambient temperature and below the temperature at which RuO4 decomposes into RuO2 (the temperature varies according to the solvent). Then the mixture is allowed to warm above the temperature at which RuO4 decomposes into RuO2 in the particular solvent and in the presence of the aerogel structure (In the presence of a substrate such as an aerogel, RuO4 decomposes at a lower temperature than it does in the absence of the substrate.). The rate of warming of the mixture is controlled so that the mixture has time to completely infiltrate the aerogel before the RuO4 decomposes. In this way, when the RuO4 decomposes, it forms a deposit on the inner and outer surfaces of the aerogel. (If the warming proceeds too quickly, newly formed RuO2 simply precipitates directly out of solution onto the bottom of the reaction vessel.) An electrically connected deposit is achieved by selecting a concentration of the RuO4 in the nonpolar solvent and a volume of the solution that is high enough so that when RuO2 becomes deposited onto the surfaces of the aerogel, a sufficient amount of RuO2 is present so that individual deposits are in electrical contact with each other. As used herein, the term xe2x80x9celectrically connectedxe2x80x9d means that for the most part, individual deposits throughout the entire aerogel structure are in electrical contact with each other, notwithstanding that there may inevitably be a few scattered or isolated deposits of RuO2 within the aerogel that are isolated or out of contact.
In the processes described herein, pentane is the preferred nonpolar solvent. Pentane has a lower freezing temperature (xe2x88x92129.7xc2x0 C.) than water and RuO4 is quite soluble in pentane. There is a dramatic decrease in RuO4 solubility with increasing temperature between xe2x88x9278xc2x0 C. and room temperature that leads to efficient deposition of Ru oxide from a pentane solution. When the temperature is raised slowly, RuO2 preferentially forms on the aerogel surfaces. Optimally, the ratio of the amount of substrate to RuO2 is high enough that all of the RuO2 is deposited within the sample and none is wasted by precipitating outside the substrate as RuO2 powder, yet low enough that there is sufficient RuO2 to form a fully connected network throughout the aerogel.
A typical process of making an electrically conductive composite may be described as follows: Briefly, a piece of silica aerogel (about 0.25 cm3) is placed in a vacuum-tight flask, evacuated to 5xc3x9710xe2x88x926 Torr, saturated with pentane vapors at ambient temperature, and cooled to xe2x88x9278xc2x0 C. (Solution extraction is used to exchange RuO4 in aqueous solution (10 mL of 0.5 wt % RuO4) into about 10 mL of pentane solution.) The RuO4 pentane solution is added to the flask and all but about 3 mL of the pentane is removed by distillation. The flask is allowed to warm gradually to room temperature over a period of about two days. Based on intermittent observations, the aerogel changes from transparent to black at about xe2x88x9235xc2x0 C., corresponding to the conversion of RuO4 to RuO2. The flask is held at room temperature for more than 12 hours, then cooled again to xe2x88x9278xc2x0 C. and the remaining pentane is distilled off. Approximately 90 to 100 wt % of the Ru in solution is deposited on the aerogel surfaces as RuO2, and about 10 to 0 wt % of the Ru in solution precipitates directly from solution as ruthenium dioxide powder. The identity of the deposit as RuO2 can be confirmed by microprobe Raman spectroscopy. Electrical conductivity of the deposit through the interior of the aerogel, and not just along the external edges of the aerogel structure can be confirmed by 2-point probe measurements across the face of a bisected cylindrical monolith of the aerogel. Typical composites have been shown to have resistivities of about 1-10 Mohms for a 0.3 cm thick sample. The resistance is decreased by two to three orders of magnitude by heating the composite in flowing oxygen or air to about 140-150xc2x0 C. This mild heat treatment increases the area of contact between deposited particles and, as confirmed by transmission electron microscopy, converts the deposited ruthenium oxide from amorphous to crystalline. (Increasing the annealing temperature to above about 200-250xc2x0 C. leads to a decrease in electrical conductivity, presumably due to grain-size coarsening. The exact temperature at which this decrease in electrical conductivity begins to occur varies with the rate of heating.) Small angle neutron scattering confirms observations made by transmission electron microscopy that the deposits of RuO2 conform to the morphology of the silica surface and do not form particles that fill the mesoporous volume of the aerogel.
Having described the invention, the following examples are given to illustrate specific applications of the invention, including the best mode now known to perform the invention. These specific examples are not intended to limit the scope of the invention described in this application.